The present invention relates to an apparatus for forming a coating film on a substrate to be treated such as a semiconductor wafer or an LCD substrate (a glass substrate for a liquid crystal display) by applying a coating solution, for example, a resist solution or the like thereto, and a coating unit.
In fabrication processes of a semiconductor device or an LCD, a resist pattern for a substrate to be treated is formed by a technology called photolithography. An apparatus for forming the resist pattern is composed by connecting an aligner to a coating and developing system provided with a cassette mounting section for carrying a wafer into/from a cassette, a coating unit for applying a resist solution, thermal treatment units for performing treatments such as heating and cooling, a developing unit for performing development, a main arm for transferring the wafer and so on.
Conventionally, the resist solution has been applied in the above-described coating unit by a so-called spin coating method. This is a method by which the resist solution is spread by centrifugal force of the wafer so that a solution film is formed over the entire wafer by providing a rotatable spin chuck in a cup surrounding the sides of the substrate over the entire circumference thereof, horizontally suction-holding the wafer by the spin chuck, and rotating the wafer while supplying the resist solution to the wafer from a nozzle above the central portion of the wafer.
Incidentally, a line width of the resist pattern to be formed is proportional to a film thickness of a resist film and an exposure wavelength. Accordingly, formation of thinner films is attempted by increasing the rotational frequency of the wafer in the spin coating method since it is necessary to make a solution film thinner to a minimum in order to cope with growing demands for a finer pattern in recent years.
However, since the wafer is rotated at a high speed in the above-described method, there is a problem that a circumferential speed of the inner circumferential portion of the wafer becomes high compared with that of the outer circumferential portion thereof, whereby air turbulence occurs at the outer peripheral portion of the wafer particularly when the wafer is upsized. The turbulence becomes a factor of impairing the formation of finer patterns since it changes the film thickness, bringing about the non-uniformity in film thickness of the entire wafer.
Further, the resist solution is spread to be blown off from the central portion of the wafer toward the peripheral portion thereof, causing a big waste of resist solution since it scatters from the peripheral portion thereof to the side of the cup. In addition, there has been a problem that the resist solution applied to portions except for a circuit formation region such as the peripheral portion or the resist solution adhered to the cup due to the scatter is hardened, which results in a cause of particles.
In view of such circumstances, a method independent of the spin coating method has been examined. According to this method, a resist solution is supplied to a wafer W in a manner of so-called single stroke by reciprocating a nozzle N in an X-direction and at the same time intermittently moving the wafer W in a Y-direction while supplying the resist solution RE from a discharge hole having a fine diameter of the nozzle N provided above the wafer W as shown in FIG. 34. Incidentally, it is preferable that the portions except for the circuit formation region on the wafer W are covered with a mask to prevent the resist solution from adhering to the periphery or the back surface of the wafer W in this case.
The wafer W is not rotated in this method, whereby the inconvenience as described above is resolved and application can be performed without causing waste. However, a thinner which dissolves a resist constituent is not shaken off unlike in the spin coating and remains intact on the wafer. For example, although no more than about 10% of the thinner remains when the spin coating is performed, substantially 100% of the thinner remains in the manner of single stroke. Therefore, when the wafer is transferred to a heating plate and undergoes drying after being applied with the resist solution, it requires long time to volatilize the thinner, and an amount of volatilization varies within the wafer since it is hard to evenly conduct heat to the resist solution, whereby the uniformity in film thickness of the film to be obtained is deteriorated even if the resist solution is uniformly applied purposely. Further, variations in the amount of volatilization within wafer become wide while the wafer is transferred to the heating plate by a main arm since the amount of volatilization is large on the wafer after being applied with the resist solution, also resulting in a factor which deteriorates the uniformity in film thickness of the resist film.